Fibronectin Glomerulopathy Without Typical Renal Biopsy Features in a 4-Year-Old Girl with Incidentally Discovered Proteinuria and a G417V FN1 Gene Mutation.

Fibronectin glomerulopathy (FG) is caused by fibronectin 1 (FN1) gene mutations. A renal biopsy was performed on a 4-year-old girl with incidentally discovered proteinuria (150 mg/dL); her family history of renal disease was negative. Markedly enlarged glomeruli (mean glomerular diameter: 196 μm; age-matched controls: 140 μm), α-SMA-positive and Ki-67-positive mesangial cell proliferation (glomerular proliferation index 1.76), the mild expansion of mesangial areas, no immune or electron-dense deposits, normal glomerular basement membrane, and diffusely effaced foot processes were observed. Genetic testing identified a de novo heterozygous mutation (Gly417Val) in the collagen-binding site of the FN II-2 domain, prompting fibronectin immunostaining. Strong mesangial positivity was noted, hence FG was diagnosed. The follow-up period of 29 months revealed nephrotic range proteinuria, intermittent microhematuria, glomerular hyperfiltration, and preserved renal function. The biopsy features of early childhood-onset FG were compared to a case of FG with a lobular pattern diagnosed in a 44-year-old patient with undulating proteinuria, microhematuria, hypertension known for a year, and a positive family history. Early childhood-onset FG was characterized by glomerular enlargement, mesangial proliferation, and no changes that suggested fibronectin deposition disease. In summary, the novel aspects of the case were that the mutation was located at the collagen-binding site of the FN1 gene, not identified earlier, and the histologic spectrum of FG was expanded by the observed mesangial proliferative pattern and striking glomerulomegaly. Now, FG should also be considered among the monogenic causes of proteinuric kidney diseases in pediatric nephrology practice.